When Sixteen Horses Failed to Separate Two Hollow Spheres
In 1654, German scientist Otto von Guericke demonstrated the immense power of atmospheric pressure to Emperor Ferdinand III. He joined two hollow copper hemispheres together with a greased leather seal and pumped the air out from the inside. Two teams of eight heavy draft horses pulling in opposite directions could not tear the spheres apart. Only when Guericke turned a valve to let air back in did the hemispheres fall apart under their own weight.
The Spectacle at Regensburg
In May 1654, the German polymath and mayor of Magdeburg, Otto von Guericke, staged a public demonstration before Emperor Ferdinand III and the assembled dignitaries of the Imperial Diet at Regensburg. Guericke brought two large, hollow copper hemispheres designed with mating rims. Between the two metal edges, he placed a ring of leather soaked in oil and wax to create an airtight seal. Once the hemispheres were pressed together, Guericke attached a mechanical suction pump of his own invention to a valve on one of the shells and pumped out the air inside.
When the evacuation was complete, Guericke closed the stopcock and hitched two teams of eight heavy draft horses to rings mounted on opposite sides of the sphere. Urged forward in opposite directions, the sixteen horses strained against the harness, but the copper halves remained locked together as though welded into a single solid body. Only when Guericke stepped forward and turned the brass valve, allowing ambient air to rush back inside the hollow cavity, did the hemispheres separate cleanly and fall apart under their own weight.
The theatrical display astonished the imperial court, transforming what might have been an abstract academic debate into a visible, tactile reality. Guericke had not merely shown that a vacuum could exist inside an enclosed vessel; he had proved that the ambient air surrounding the Earth pressed against every surface with immense, previously unrecognized physical power.
The Mechanics of Atmospheric Pressure
The inability of the horses to separate the spheres was not caused by a mysterious suction pulling from within, but by the relentless weight of the atmosphere pushing from without. Earth is enveloped by an ocean of air extending miles upward. At sea level, this column of gas exerts an ambient pressure of approximately 14.7 pounds per square inch (roughly 101 kilopascals) in all directions against every exposed surface.
Under normal conditions, hollow objects do not collapse because the air inside pushes outward with the exact same pressure that the outside air pushes inward, maintaining a state of mechanical equilibrium. When Guericke pumped the air out of the Magdeburg hemispheres, he removed that internal counter-pressure. The external atmosphere continued to press against the outer surface of the copper shells, holding them together with a force proportional to the surface area of the circular cross-section defined by their joined rims.
For hemispheres roughly 50 centimeters in diameter, the cross-sectional area meant that the atmosphere clamped the two halves together with a force equivalent to several thousand pounds. Because the external air pressed inward evenly from all directions, pulling from one side against an equal pull from the other simply forced the draft horses to struggle against the weight of the sky itself.
Overturning the Horror Vacui
Guericke's demonstration struck directly at one of the central tenets of classical natural philosophy. For nearly two millennia, mainstream European science had followed Aristotle's doctrine of 'horror vacui'—the assertion that nature abhors a vacuum. Aristotelian physics maintained that space could never be truly empty, arguing that if an empty void were ever created, surrounding matter would instantly rush in to fill it at infinite speed.
Cracks in this ancient consensus began to form in the early 1640s when Italian physicist Evangelista Torricelli, a student of Galileo Galilei, inverted a glass tube filled with mercury into an open basin. The column of mercury fell only partially, leaving an empty space at the top of the sealed tube. Torricelli correctly deduced that the column was held aloft not by nature's refusal to permit a void, but by the weight of the atmosphere pressing down on the mercury in the open dish below.
While Torricelli's barometer demonstrated atmospheric pressure on a miniature scale, skeptics continued to argue that the space above the mercury was filled with invisible vapors or subtle spirits rather than being a true vacuum. Guericke realized that settling the question required a macroscopic demonstration that could physically manipulate large volumes of evacuated space.
The Invention of the Piston Air Pump
Achieving a reliable vacuum required overcoming severe engineering obstacles. Before designing the copper hemispheres, Guericke spent years experimenting with various containers and pumping mechanisms. His earliest attempts involved taking a wooden beer cask, filling it with water, and attempting to pump the water out using a modified fire-fighting pump. The experiments failed repeatedly because the immense external air pressure forced air through the pores and joints of the wood, causing the casks to leak or suddenly implode.
Recognizing that wood was too porous, Guericke commissioned large hollow copper spheres and developed a purpose-built mechanical vacuum pump. His apparatus utilized a cylinder, a tight-fitting piston, and leather flap valves lubricated with water and oil. By repeatedly drawing the piston back and expelling the trapped air through one-way valves, Guericke succeeded in creating a practical, man-made partial vacuum inside a metallic chamber.
The development of this pump represented a major breakthrough in experimental technology. Rather than relying purely on natural phenomena or passive observation, Guericke gave scientists an active tool with which they could alter the physical environment inside an apparatus, test materials in the absence of air, and systematically measure the consequences of changing atmospheric conditions.
From Regensburg to the Royal Society
Guericke repeated and refined his demonstration several times across the 1650s and 1660s. In 1656, he staged an even larger performance in his home city of Magdeburg, and later demonstrated the experiment in Berlin for Frederick William, Elector of Brandenburg, using up to twenty-four horses. Word of these spectacular trials spread quickly through early modern scientific networks.
In 1657, the Jesuit scholar Gaspar Schott published an account of Guericke’s vacuum pump and the Magdeburg experiments in his treatise 'Mechanica Hydraulico-Pneumatica'. Schott's book was read by natural philosophers across Europe, directly inspiring Robert Boyle and Robert Hooke in England. Hooke redesigned Guericke's air pump into a more practical glass-receiver apparatus, which Boyle used to conduct the landmark pneumatic experiments that produced Boyle's Law.
Guericke eventually published his own comprehensive treatise on the subject in 1672, titled 'Experimenta Nova (ut vocantur) Magdeburgica de Vacuo Spatio'. The book documented not only the famous hemisphere experiments and the details of his vacuum pumps, but also his pioneer observations on electrostatic repulsion, the behavior of sound in a vacuum, and the preservation of biological materials in the absence of air.
The Legacy of the Magdeburg Experiment
The Magdeburg hemispheres remain one of the defining symbols of the Scientific Revolution. Guericke demonstrated that scientific truths could be settled decisively through public, repeatable experimentation rather than philosophical disputation. By making the invisible atmosphere visible through mechanical force, he bridged the gap between abstract mathematical physics and tangible, macroscopic engineering.
Beyond its philosophical impact, the mastery of atmospheric pressure and vacuums laid the direct foundation for the industrial era. The realization that condensing steam creates a partial vacuum, allowing the outside atmosphere to drive a heavy piston down with great force, was the core operating principle behind Thomas Newcomen’s atmospheric engine in the early eighteenth century, which kicked off the development of modern steam power.
What began as a dramatic imperial demonstration in 1654 ultimately transformed humanity's understanding of the air we inhabit. The sixteen horses straining against the hollow copper sphere did not simply prove the strength of a seal; they revealed that life on Earth exists at the bottom of a vast, heavy, and energetic sea of air.
Key takeaways
•The Magdeburg hemispheres were held together entirely by external atmospheric pressure pressing against a circular cross-section, not by internal suction.
•Otto von Guericke built the first functional mechanical piston vacuum pump, enabling deliberate and repeatable pneumatic experiments.
•The 1654 demonstration decisively disproved the classical Aristotelian doctrine of 'horror vacui' (that nature abhors a vacuum).
•Guericke's experiments directly inspired Robert Boyle and Robert Hooke, laying the experimental foundation for modern gas physics and atmospheric steam engines.